Phase-based operation of devices on a polyphase electric distribution system

ABSTRACT

In one embodiment, a device in a computer network monitors an alternating-current (AC) waveform of an electrical power source at the device, where the power source is part of a polyphase power source system. Once the device determines a particular phase of the polyphase power source system at the device, then the device joins a directed acyclic graph (DAG) specific to the particular phase. In another embodiment, a device detects a time of a zero crossing of the AC waveform, and may then determine a particular phase of the polyphase power source system at the device based on the time of the zero crossing relative to a corresponding location within a frequency hopping superframe of the computer network.

RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 13/043,213 filed Mar. 8, 2011, by Shaffer, et al.,the contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates generally to computer networks, and, moreparticularly, to phase-based operation of devices and directed acyclicgraph (DAG) establishment in electric transmission grids.

BACKGROUND

Electric power is generally transmitted from generation plants to endusers (industries, corporations, homeowners, etc.) via a transmissionand distribution grid consisting of a network of power stations,transmission circuits, and substations interconnected by powerlines.Once at the end users, electricity can be used to power any number ofdevices. The transfer of alternating-current (AC) electric power to theend users most frequently takes the form of three-phase electric power,where three current waveforms are produced that are generally equal inmagnitude and 120° out of phase to each other. If the load on athree-phase system is balanced equally among the phases, no currentflows through a neutral point, which is an important design aspect ofthe electric grid, allowing for efficient use of transformer capacity,reduced materials (e.g., size of a neutral conductor), etc. However,there are many factors that may create imbalance between the phases,such as excess load usage, downed powerlines, etc.

The topology of the electric transmission grid typically considers thebalancing of the three-phase system, such that each end user (and thusthe end user's devices) is attached to the grid via a particular phase'scurrent waveform (though certain customers may be connected to two orthree phases of the grid). Most often, however, the end users, and morespecifically the end users' devices, are unaware of which phase they areoperating upon. Accordingly, there is no phase-based control of thedevices beyond planning the physical topology of the grid, which isdifficult (and often expensive) to change once it is installed.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments herein may be better understood by referring to thefollowing description in conjunction with the accompanying drawings inwhich like reference numerals indicate identically or functionallysimilar elements, of which:

FIG. 1 illustrates an example computer network;

FIG. 2 illustrates an example network device/node;

FIG. 3 illustrates an example message;

FIG. 4 illustrates an example polyphase electric distribution system;

FIGS. 5A-B illustrate example phase representations of the polyphaseelectric distribution system;

FIG. 6 illustrates an example alternating-current (AC) waveform for thepolyphase electric distribution system;

FIG. 7 illustrates an example frequency hopping sequence withsuperframes;

FIGS. 8A-B illustrate example zero crossings of the AC waveform withrelation to the frequency hopping superframes;

FIG. 9 illustrates example phase-based directed acyclic graphs (DAGs) inthe computer network of FIG. 1;

FIG. 10 illustrates an example demand response (DR) message on a DAG;

FIG. 11 illustrates a change in the example polyphase electricdistribution system of FIG. 4;

FIG. 12 illustrates a corresponding change in the example phase-basedDAGs in the computer network of FIG. 9;

FIG. 13 illustrates an example simplified procedure for creatingphase-based DAGs; and

FIG. 14 illustrates an example simplified procedure for determining aparticular phase of a device in a polyphase system based on its relationto frequency hopping.

DESCRIPTION OF EXAMPLE EMBODIMENTS Overview

According to one or more embodiments of the disclosure, a device in acomputer network monitors an alternating-current (AC) waveform of anelectrical power source at the device, where the power source is part ofa polyphase power source system. Once the device determines a particularphase (or phases) of the polyphase power source system at the devicebased on the AC waveform, then the device joins a directed acyclic graph(DAG) specific to the particular phase.

According to one or more additional embodiments of the disclosure, adevice in a frequency hopping computer network monitors an AC waveformof an electrical power source at the device, where the power source ispart of a polyphase power source system. The device detects a time of azero crossing of the AC waveform, and may then determine a particularphase of the polyphase power source system at the device based on thetime of the zero crossing relative to a corresponding location within afrequency hopping superframe of the computer network.

Description

A computer network is a geographically distributed collection of nodesinterconnected by communication links and segments for transporting databetween end nodes, such as personal computers and workstations, or otherdevices, such as sensors, utility meters, etc. Many types of networksare available, with the types ranging from local area networks (LANs) towide area networks (WANs). LANs typically connect the nodes overdedicated private communications links located in the same generalphysical location, such as a building or campus. WANs, on the otherhand, typically connect geographically dispersed nodes overlong-distance communications links, such as common carrier telephonelines, optical lightpaths, synchronous optical networks (SONET),synchronous digital hierarchy (SDH) links, or Powerline Communications(PLC) such as IEEE 61334, IEEE P1901.2, and others. In addition, aMobile Ad-Hoc Network (MANET) is a kind of wireless ad-hoc network,which is generally considered a self-configuring network of mobileroutes (and associated hosts) connected by wireless links, the union ofwhich forms an arbitrary topology.

Smart object networks, in particular, are a specific type of networkhaving spatially distributed autonomous devices such as sensors,actuators, etc. For example, sensor networks may cooperatively monitorphysical or environmental conditions at different locations, such as,e.g., energy/power consumption, resource consumption, etc. Another typeof smart object includes actuators, e.g., responsible for turning on/offan engine or perform any other actions. Generally, smart object networksmay include any type of device that is able to communicate informationon a computer network, such as household appliances (air conditioners,refrigerators, lights, etc.), industrial devices (heating, ventilating,and air conditioning (HVAC), pumps, motors, etc.), and other “smart”devices.

That is, smart object networks are typically interconnected by acomputer network, such as wireless networks, though wired connectionsare also available. For instance, each smart device (node) in a smartobject network may generally be equipped with a radio transceiver orother communication port, a microcontroller, and an energy source, suchas a battery (or, in particular to the embodiments herein, atransmission grid power source). Typically, size and cost constraints onsensor nodes result in corresponding constraints on resources such asenergy, memory, computational power and bandwidth. Correspondingly, areactive routing protocol may, though need not, be used in place of aproactive routing protocol for sensor networks.

FIG. 1 is a schematic block diagram of an example computer network 100illustratively comprising “smart” nodes/devices 200 (e.g., labeled asshown, “A” through “J,” and described in FIG. 2 below) interconnected byvarious methods of communication, such as links 105. For instance, thelinks between the nodes may be wired links or may comprise a wirelesscommunication medium, where certain nodes 200 may be in communicationwith other nodes 200, e.g., based on distance, signal strength, currentoperational status, location, etc. Those skilled in the art willunderstand that any number of nodes, devices, links, etc. may be used inthe computer network, and that the view shown herein is for simplicity.

Note that one example communication link 105 is powerline communicationlinks, which often exhibit, in many ways, similar properties as wireless(radio frequency or “RF”) links, such as in terms of lossiness,collisions, etc. Also, as will be understood by those skilled in theart, powerline communication systems are multi-hop systems, where eventhough the underlying transmission medium (the powerline) interconnectsall of the nodes (e.g., like a broadcast domain), nodes in a PLC networkcommunicate with each other over the transmission medium through othernodes relaying/routing messages.

Data packets 140 (e.g., traffic and/or messages) may be exchanged amongthe nodes/devices of the computer network 100 using predefined networkcommunication protocols such as the Transmission ControlProtocol/Internet Protocol (TCP/IP), User Datagram Protocol (UDP),Multi-Protocol Label Switching (MPLS), various proprietary protocols,etc. In this context, a protocol consists of a set of rules defining howthe nodes interact with each other. In addition, packets within thenetwork 100 may be transmitted in a different manner depending upondevice capabilities, such as source routed packets.

FIG. 2 is a schematic block diagram of an example node/device 200 thatmay be used with one or more embodiments described herein, e.g., as asmart device in the network 100. The device may comprise a networkinterface 210, a processor 220 (e.g., an 8-64 bit microcontroller), anda memory 240 interconnected by a system bus 250. Notably, the device mayalso be powered by a power supply 260, such as a battery, or, morespecifically with regard to one or more embodiments herein, a “plug-in”power supply.

The network interface 210 contains the mechanical, electrical, andsignaling circuitry for communicating data over physical and/or wirelesslinks coupled to the network 100. The network interface may beconfigured to transmit and/or receive data using a variety of differentcommunication protocols, including, inter alia, TCP/IP, UDP, wirelessprotocols (e.g., IEEE Std. 802.15.4, WiFi, Bluetooth®,), Ethernet,powerline communication (PLC) protocols, broadband over power lines(BPL), etc. Note that certain devices may have two different types ofnetwork connections 210. For instance, devices may have one or moreinterfaces used to communicate with other devices within the computernetwork (e.g., a mesh cell), and for certain other devices (“root”devices), another interface may be used as a WAN uplink networkinterface between the root node and, for example, a head-end devicelocated through the WAN.

The memory 240 comprises a plurality of storage locations that areaddressable by the processor 220 for storing software programs and datastructures associated with the embodiments described herein. Notably,certain devices may have limited memory or no memory (e.g., no memoryfor storage other than for programs/processes operating on the device).The processor 220 may comprise necessary elements or logic adapted toexecute the software programs and manipulate the data structures 245. Anoperating system 242, portions of which are typically resident in memory240 and executed by the processor, functionally organizes the device by,inter alia, invoking operations in support of software processes and/orservices executing on the device. These software processes and/orservices may comprise routing process/services 244, which may include anillustrative directed acyclic graph (DAG) process 246. Also, anillustrative phase detection process 247 may also be present in memory240, for use as described herein, as well as a demand response (DR)process 248.

It will be apparent to those skilled in the art that other processor andmemory types, including various computer-readable media, may be used tostore and execute program instructions pertaining to the techniquesdescribed herein. Also, while the description illustrates variousprocesses, it is expressly contemplated that various processes may beembodied as modules configured to operate in accordance with thetechniques herein (e.g., according to the functionality of a similarprocess).

Routing process (services) 244 contains computer executable instructionsexecuted by the processor 220 to perform functions provided by one ormore routing protocols, such as proactive or reactive routing protocolsas will be understood by those skilled in the art. These functions may,on capable devices, be configured to manage a routing/forwarding tablecontaining, e.g., data used to make routing/forwarding decisions. Inparticular, in proactive routing, connectivity is discovered and knownprior to computing routes to any destination in the network, e.g., linkstate routing such as Open Shortest Path First (OSPF), orIntermediate-System-to-Intermediate-System (ISIS), or Optimized LinkState Routing (OLSR). Reactive routing, on the other hand, discoversneighbors (i.e., does not have an a priori knowledge of networktopology), and in response to a needed route to a destination, sends aroute request into the network to determine which neighboring node maybe used to reach the desired destination. Example reactive routingprotocols may comprise Ad-hoc On-demand Distance Vector (AODV), DynamicSource Routing (DSR), DYnamic MANET On-demand Routing (DYMO), etc.Notably, on devices not capable or configured to store routing entries,routing process 244 may consist solely of providing mechanisms necessaryfor source routing techniques. That is, for source routing, otherdevices in the network can tell the less capable devices exactly whereto send the packets, and the less capable devices simply forward thepackets as directed.

Low power and Lossy Networks (LLNs), e.g., certain smart objectnetworks, may be used in a myriad of applications such as for “SmartGrid” and “Smart Cities.” A number of challenges in LLNs have beenpresented, such as:

1) Links are generally lossy, such that a Packet Delivery Rate/Ratio(PDR) can dramatically vary due to various sources of interferences,e.g., considerably affecting the bit error rate (BER);

2) Links are generally low bandwidth, such that control plane trafficmust generally be bounded and negligible compared to the low rate datatraffic;

3) There are a number of use cases that require specifying a set of linkand node metrics, some of them being dynamic, thus requiring specificsmoothing functions to avoid routing instability, considerably drainingbandwidth and energy;

4) Constraint-routing may be required by some applications, e.g., toestablish routing paths that will avoid non-encrypted links, nodesrunning low on energy, etc.;

5) Scale of the networks may become very large, e.g., on the order ofseveral thousands to millions of nodes; and

6) Nodes may be constrained with a low memory, a reduced processingcapability, a low power supply (e.g., battery).

In other words, LLNs are a class of network in which both the routersand their interconnects are constrained: LLN routers typically operatewith constraints, e.g., processing power, memory, and/or energy(battery), and their interconnects are characterized by, illustratively,high loss rates, low data rates, and/or instability. LLNs are comprisedof anything from a few dozen and up to thousands or even millions of LLNrouters, and support point-to-point traffic (between devices inside theLLN), point-to-multipoint traffic (from a central control point to asubset of devices inside the LLN) and multipoint-to-point traffic (fromdevices inside the LLN towards a central control point).

An example protocol specified in an Internet Engineering Task Force(IETF) Internet Draft, entitled “RPL: IPv6 Routing Protocol for LowPower and Lossy Networks”<draft-ietf-roll-rpl-18> by Winter, at al.(Feb. 4, 2011 version), provides a mechanism that supportsmultipoint-to-point (MP2P) traffic from devices inside the LLN towards acentral control point (e.g., LLN Border Routers (LBRs) or “rootnodes/devices” generally), as well as point-to-multipoint (P2MP) trafficfrom the central control point to the devices inside the LLN (and alsopoint-to-point, or “P2P” traffic). RPL (pronounced “ripple”) maygenerally be described as a distance vector routing protocol that buildsa Directed Acyclic Graph (DAG) for use in routing traffic/packets 140,in addition to defining a set of features to bound the control traffic,support repair, etc. Notably, as may be appreciated by those skilled inthe art, RPL also supports the concept of Multi-Topology-Routing (MTR),whereby multiple DAGs can be built to carry traffic according toindividual requirements.

Note that while LLNs and RPL are described herein, the techniquesdescribed herein may be utilized with any type of computer network(smart object network) and any suitable type of DAG creation protocol.

A DAG is a directed graph having the property that all edges areoriented in such a way that no cycles (loops) are supposed to exist. Alledges are contained in paths oriented toward and terminating at one ormore root nodes (e.g., “clusterheads or “sinks”), often to interconnectthe devices of the DAG with a larger infrastructure, such as theInternet, a wide area network, or other domain. In addition, aDestination Oriented DAG (DODAG) is a DAG rooted at a singledestination, i.e., at a single DAG root with no outgoing edges. A“parent” of a particular node within a DAG is an immediate successor ofthe particular node on a path towards the DAG root. Note also that atree is a kind of DAG, where each device/node in the DAG generally hasone parent or one preferred parent. DAGs may generally be built based onan Objective Function (OF). The role of the Objective Function isgenerally to specify rules on how to build the DAG (e.g. number ofparents, backup parents, etc.).

In addition, one or more metrics/constraints may be advertised by therouting protocol to optimize the DAG against. Also, the routing protocolallows for including an optional set of constraints to compute aconstrained path, such as if a link or a node does not satisfy arequired constraint, it is “pruned” from the candidate list whencomputing the best path. (Alternatively, the constraints and metrics maybe separated from the OF.) Additionally, the routing protocol mayinclude a “goal” that defines a host or set of hosts, such as a hostserving as a data collection point, or a gateway providing connectivityto an external infrastructure, where a DAG's primary objective is tohave the devices within the DAG be able to reach the goal. In the casewhere a node is unable to comply with an objective function or does notunderstand or support the advertised metric, it may be configured tojoin a DAG as a leaf node. As used herein, the various metrics,constraints, policies, etc., are considered “DAG parameters.”

Illustratively, example metrics used to select paths (e.g., preferredparents) may comprise cost, delay, latency, bandwidth, estimatedtransmission count (ETX), etc., while example constraints that may beplaced on the route selection may comprise various reliabilitythresholds, restrictions on battery operation, multipath diversity,bandwidth requirements, transmission types (e.g., wired, wireless,etc.). The OF may provide rules defining the load balancingrequirements, such as a number of selected parents (e.g., single parenttrees or multi-parent DAGs). Notably, an example for how routing metricsand constraints may be obtained may be found in an IETF Internet Draft,entitled “Routing Metrics used for Path Calculation in Low Power andLossy Networks”<draft-ietf-roll-routing-metrics-18> by Vasseur, et al.(Feb. 22, 2011 version). Further, an example OF (e.g., a default OF) maybe found in an IETF Internet Draft, entitled “RPL Objective Function0”<draft-ietf-roll-of0-05> by Thubert (Jan. 5, 2011 version).

Building a DAG may utilize a discovery mechanism to build a logicalrepresentation of the network, and route dissemination to establishstate within the network so that routers know how to forward packetstoward their ultimate destination. Note that a “router” refers to adevice that can forward as well as generate traffic, while a “host”refers to a device that can generate but does not forward traffic. Also,a “leaf” may be used to generally describe a non-router that isconnected to a DAG by one or more routers, but cannot itself forwardtraffic received on the DAG to another router on the DAG. Controlmessages may be transmitted among the devices within the network fordiscovery and route dissemination when building a DAG.

According to the illustrative RPL protocol, a DODAG Information Object(DIO) is a type of DAG discovery message that carries information thatallows a node to discover a RPL Instance, learn its configurationparameters, select a DODAG parent set, and maintain the upward routingtopology. In addition, a Destination Advertisement Object (DAO) is atype of DAG discovery reply message that conveys destination informationupwards along the DODAG so that a DODAG root (and other intermediatenodes) can provision downward routes. A DAO message includes prefixinformation to identify destinations, a capability to record routes insupport of source routing, and information to determine the freshness ofa particular advertisement. Notably, “upward” or “up” paths are routesthat lead in the direction from leaf nodes towards DAG roots, e.g.,following the orientation of the edges within the DAG. Conversely,“downward” or “down” paths are routes that lead in the direction fromDAG roots towards leaf nodes, e.g., generally going in the oppositedirection to the upward messages within the DAG.

Generally, a DAG discovery request (e.g., DIO) message is transmittedfrom the root device(s) of the DAG downward toward the leaves, informingeach successive receiving device how to reach the root device (that is,from where the request is received is generally the direction of theroot). Accordingly, a DAG is created in the upward direction toward theroot device. The DAG discovery reply (e.g., DAO) may then be returnedfrom the leaves to the root device(s) (unless unnecessary, such as forUP flows only), informing each successive receiving device in the otherdirection how to reach the leaves for downward routes. This processhelps build routing tables to send downward messages to any node in theDAG and not only to the leafs. Nodes that are capable of maintainingrouting state may aggregate routes from DAO messages that they receivebefore transmitting a DAO message. Nodes that are not capable ofmaintaining routing state, however, may attach a next-hop parentaddress. The DAO message is then sent directly to the DODAG root thatcan in turn build the topology and locally compute downward routes toall nodes in the DODAG. Such nodes are then reachable using sourcerouting techniques over regions of the DAG that are incapable of storingdownward routing state.

FIG. 3 illustrates an example simplified control message format 300 thatmay be used for discovery and route dissemination when building a DAG,e.g., as a DIO or DAO. Message 300 illustratively comprises a header 310with one or more fields 312 that identify the type of message (e.g., aRPL control message), and a specific code indicating the specific typeof message, e.g., a DIO or a DAO (or a DAG Information Solicitation).Within the body/payload 320 of the message may be a plurality of fieldsused to relay the pertinent information. In particular, the fields maycomprise various flags/bits 321, a sequence number 322, a rank value323, an instance ID 324, a DODAG ID 325, and other fields, each as maybe appreciated in more detail by those skilled in the art. Further, forDAO messages, additional fields for destination prefixes 326 and atransit information field 327 may also be included, among others (e.g.,DAO_Sequence used for ACKs, etc.). For either DIOs or DAOs, one or moreadditional sub-option fields 328 may be used to supply additional orcustom information within the message 300. For instance, an objectivecode point (OCP) sub-option field may be used within a DIO to carrycodes specifying a particular objective function (OF) to be used forbuilding the associated DAG. Alternatively or in addition, sub-optionfields may be used to carry other certain information within a message300, such as indications, requests, capabilities, lists, etc., as may bedescribed herein, e.g., in one or more type-length-value (TLV) fields.For instance, sub-option fields may be used to carry phase-basedinformation as described below.

Phased-Based Operation of Devices

As noted above, electric power is generally transmitted from generationplants to end users (industries, commercial, residential, etc.) via atransmission grid consisting of a network of power stations,transmission circuits, and substations interconnected by power lines.Once at the end users, electricity can be used to power any number ofdevices, such as devices 200. The transmission and distribution ofalternating-current (AC) electric power to the end users most frequentlytakes the form of polyphase electric power, a common form of which beingthree-phase electric power. For smaller customers (e.g., households)usually a single phase is taken to the property. For largerinstallations (large houses, buildings), all three phases may be takento a distribution panel, from which both single and multi (two orthree-phase) circuits may be fed.

FIG. 4 illustrates an example electric power transmission anddistribution grid 400 to the example devices A-J of FIG. 1, above. (Notethat although the terminology herein often refers to either“transmission” or “distribution,” those skilled in the art willrecognize that the embodiments herein may broadly cover both thetransmission and distribution portion of the grid.) For instance, adistribution center 410 (e.g., generation plant, transformer station,etc.) supplies electricity over a plurality of transmission lines 415 tothe devices 200. In the embodiments herein, the supplied electricity ispart of a polyphase source system, where a plurality of phases (e.g.,three) are transmitted onto the lines 415 to the devices, such that eachdevice is generally attached to a particular phase of the electric grid.As shown, electrical power of three phases, L1, L2, and L3, is suppliedto the devices A-J (a neutral/ground may be shared by the phases).Notably, the view shown herein is vastly simplified, as each phase maygenerally be used to power entire buildings, neighborhoods, etc, and mayalso supply power to many (e.g., tens, hundreds, thousands) of deviceswithin those establishments. Also, while the view shown herein isgenerally arbitrarily connected, phase-based distribution gridtopologies generally result in “clusters” of like-phased devices (e.g.,those within the buildings, neighborhoods, etc.).

FIG. 5A illustrates an example phase representation 500 of thetransmission (distribution) grid's electrical power. In particular,three current waveforms are illustratively produced (L1, L2, and L3)that are generally equal in magnitude and 120° out of phase to eachother. The currents returning from the end users to the supplytransformer all share the neutral wire (neutral point 505). If the loadsare evenly distributed on all three phases, as they are in FIG. 5A, thesum of the returning currents in the neutral wire is approximately zero.Any unbalanced phase loading such as in FIG. 5B, however, may result ina current 506 at the neutral point, which may cause inefficient use oftransformers, vibrations in generators, or other problems, including(but not limited to) brown-outs or black-outs in extreme cases. Thereare many factors that may create imbalance between the phases, such asexcess load usage, downed power lines, etc.

The topology of the electric transmission grid typically considers thebalancing of the three-phase system, such that each end user (and thusthe end user's devices) is attached to the grid via a particular phase'scurrent waveform. Most often, however, the end users, and morespecifically the end users' devices, are unaware of which phase they areoperating upon. In other words, the topology of the distribution grid isindependent of the topology of the computer network. Accordingly, thereis no phase-based control of the devices beyond planning the physicaltopology of the grid, which is difficult (and often expensive) to changeonce it is installed. For instance, if an imbalance is detected in thedistribution system, there is no mechanism currently available toefficiently control devices plugged in to the grid in a phase-basedmanner.

The techniques herein therefore provide for phase-based control ofnetworked devices in a polyphase electric distribution grid.Specifically, according to one or more embodiments of the disclosure asdescribed in greater detail below, a device in a computer networkmonitors an alternating-current (AC) waveform of an electrical powersource at the device. Once the device determines a particular phase ofthe polyphase power source system (electric distribution grid) at thedevice based on the AC waveform, the device may then joins a directedacyclic graph (DAG) specific to the particular phase. Control messages(e.g., demand response or “DR” messages) may then be directed to deviceson a particular DAG based on their associated phase.

Also, according to one or more additional embodiments of the disclosureas described in greater detail below, to determine the particular phase,the device may detect a time of a zero crossing of the AC waveform, andmay then determine the particular phase of the polyphase power sourcesystem at the device based on the time of the zero crossing relative toa corresponding location within a frequency hopping superframe of thecomputer network. In one or more associated embodiments, the DR messagesmay be sent out to all devices along with an indication of the intendedphase, such that each device knows its phase and thus knows whether toreact to the DR messages.

Illustratively, the techniques described herein may be performed byhardware, software, and/or firmware, such as in accordance with phasedetection process 247, which may contain computer executableinstructions executed by the processor 220 to perform functions relatingto the novel techniques described herein, e.g., in conjunction with DAGprocess 246 (creation of DAGs, joining to DAGs, etc.). For example, thetechniques herein may be treated as extensions to conventionalprotocols, such as the RPL protocol, and as such, would be processed bysimilar components understood in the art that execute the RPL protocol,accordingly.

Operationally, by monitoring the AC waveform of the electrical powersource at the device (e.g., from its power supply) phase detectionprocess 247 can determine a particular phase of the device's power basedon the AC waveform. While other techniques are possible, oneillustrative novel technique that may be used to determine the phase isby detecting a time of a zero crossing of the AC waveform, anddetermining the particular phase based on the time of the zero crossingrelative to a corresponding location within a frequency hoppingsuperframe of the computer network.

For instance, as illustrated in FIG. 6, the usual form of an AC powercircuit is a sine wave 600. In a three-phase system, three circuitconductors carry three alternating currents (of the same frequency), L1(600-1), L2 (600-2), and L3 (600-3), which reach their instantaneouspeak values at different times. For the sake of simplicity, the examplesherein ignore the neutral wire which is used in many topologies. Takingone sine wave of conductor (L1) as the reference, the other two sinewaves are delayed in time by one-third (L2) and two-thirds (L3) of onecycle of the electric current. Note that each single-phase waveform goesto zero at each moment that the voltage crosses zero. The illustrativetechnique determines these “zero crossings” 610, such as each time thewave crosses zero, or as shown, each time the wave increases throughzero (or alternatively decreases through zero).

Each device (with a phase detection process 247) can determine thetiming of its waveforms zero crossings, either 610-1, 610-2, or 610-3.With the determined zero crossing, the device then needs to correlatethis time (these times) with a global reference clock. According to theillustrative technique, each node determines on which phase it sits bycross-referencing the zero crossing with a super-channel clock used forfrequency hopping sequences.

Frequency hopping, also referred to as “frequency-hopping spreadspectrum” (FHSS) is a method of transmitting radio signals by rapidlyswitching a carrier among numerous frequency channels, e.g., using apseudorandom sequence known to both transmitter and receiver. Forexample, frequency hopping may be utilized as a multiple access methodin the frequency-hopping code division multiple access (FH-CDMA) scheme.Generally, as may be appreciated by those skilled in the art,transmission using frequency hopping is different from a fixed-frequencytransmission in that frequency-hopped transmissions are resistant tointerference and are difficult to intercept. Accordingly,frequency-hopping transmission is a useful technique for manyapplications, such as sensor networks, LLNs, military applications, etc.

In particular, as shown in FIG. 7, in frequency hopping networks, timeframes (“superframes”) are divided within a frequency hopping sequence700 into regular timeslots 710 (I, II, III, etc.), each one operating ona different frequency. A reference clock may be provided for the timeframes for an entire network (e.g., mesh/cell), and a media accesscontrol (MAC) layer of each node divides time into timeslots that arealigned with the timeslot boundary of its neighbor (e.g., parent node).Also, each timeslot 710 may be further divided into sub-timeslots 720,e.g., 6, 8, or 12 sub-timeslots within a timeslot. Illustratively, theMAC layer is in charge of scheduling the timeslot in which a packet issent, the main objective of which being randomization of thetransmission time in order to avoid collisions with neighbors' packets.

According to this illustrative phase determination technique, eachdevice may receive information about the zero crossing timing of eachphase with relation to the frequency hopping superframe 700. Forinstance, such “interpretation” information may be relayed via DIOmessage 300 down to the nodes of the network, such that each node/devicemay use this information to determine how to proceed. In accordance witha particular embodiment, for example, a DAG root may be used as areference node against which all other nodes in the DAG calibrate theirphase measurement. That is, the only thing visible to a specific node incertain embodiments is when its own power sine wave crosses zero. Inorder to bring this into perspective, the nodes may need reference zerocrossing information from a DAG root or else report their zero crossingto the root for computation, each as described below.

FIG. 8A illustrates an example correlation between zero crossings 610and the frequency hopping superframe 700. For instance, by overlayingthe waveforms 600 onto the superframe 700, it can be seen how the zerocrossings 610 correspond to a particular location (e.g., a particularsub-frame 720) within the superframe. Having the interpretationinformation above, each device can determine whether it is attached tophase L1, L2, or L3 based on when its zero crossings occur in relationto the timing of the frequency hopping superframe. Note that the zerocrossings through the frequency hopping frame occur at certain pointsbased on the phase, but that these points could happen anywhere (in amanner known for the interpretation information). The view shown hereinis merely for illustration.

It is recognized that reactive loads and capacitor banks may affect thespecific angle between the three phases. Therefore, as the load changeswith time, the specific relative zero crossing may change with time aswell. In accordance with a specific embodiment each node monitors andreports the timing of its zero crossing to the DAG root. The DAG rootaverages the reported timing from each node and keeps track of the phaseto which the node is connected. It should be also noted that the phase(zero crossings) variations are usually smaller than the nominal 120degrees between the various phases. One aspect of the above-mentionedsystem is to detect (and record) when, e.g., due to maintenance or gridrestoration, a node is switched from one phase to another phase.

Further, FIG. 8B illustrates how these zero crossings may not be thesame between frames 710, since the period of the waveform may notdirectly coincide with the period of the frames within the superframe.Accordingly, the interpretation information may include a number of zerocrossing timings throughout the superframe for each particular phase'swaveform. Alternatively or in addition, since the period of thewaveforms 600 is regular (i.e., consistent, such as 60 Hz), in oneembodiment the interpretation is merely the first zero crossing of eachwaveform, or even more simplistically, of a particular waveform (sincethe remaining waveforms are equidistant, e.g., at one-third andtwo-thirds of the first waveform's period for a three-phase system).

Note, again, that using the frequency hopping superframe is merely oneexample technique to determine the phase. Other techniques may be usedherein, such as those using zero crossings (e.g., correlated with ageneral network clock), or otherwise (e.g., using signals or pulsestransmitted and detected on each individual phase's transmission lines).

According to one or more embodiments herein, once a device knows itsphase relation, the device can join a directed acyclic graph (DAG)specific to the particular phase. FIG. 9 illustrates example DAGs thatmay be created, e.g., through the techniques described above, withinnetwork 100 of FIG. 1, such that, in essence, a special DAG is createdfor each set of nodes (e.g., meters) associated with one of the phases.For instance, for a three-phase system, three corresponding DAGs may becreated (or more, such as for general maintenance and operation).Certain links may be selected for each node to communicate with aparticular parent (and thus, in the reverse, to communicate with achild, if one exists) based on the phase of each node. That is, when achoosing a potential parent node, a child node considers the parentnode's associated phase as a DAG parameter (communicated to the childnode via messages 300, accordingly). These selected links form the DAGs900, which each extend from a root node toward one or more leaf nodes(nodes without children). Traffic/packets 140 (shown in FIG. 1) may thentraverse each of the DAGs 900 independently in either the upwarddirection toward the root or downward toward the leaf nodes.

In particular, in one or more embodiments, the specific phase detectionmay occur in concert with information from an original DAG root. Forexample, the process can start with all nodes in the network 100communicating with a single master root, e.g., node A in the topologyshown in FIG. 1 above. This initial root may select a specific phase tobe the reference phase (e.g., 600-1, L1's waveform) and informs thenodes in its DAG (routing domain/cell) regarding the offset of the zerocrossing 610-1 it sees for the reference sine wave and the super frame700. The other two root nodes (e.g., node B and node C) may each selectone of the other phases and declare (advertise) that they will serviceit. Nodes may then begin to migrate to the new DAGs (900-2 and 900-3),while certain nodes remain in the original DAG (now 900-1) as theirphase matches the phase of the original root, node A.

As noted above, the DAGs 900-1, 900-2, and 900-3 may be configured asMTR instances on the devices, such that a general, all-encompassing DAGmay be used to ensure connectivity to all of the devices in the network.Also, as an additional feature on top of the phase-specific DAGs, or asan alternative to phase-specific DAGs altogether, multicast groups maybe established for each phase, such that devices may join acorresponding multicast group specific to its phase. Similarly, avirtual local area network (VLAN) may be configured for each phase,e.g., on each DAG, as may be appreciated by those skilled on the art.

Now that each device is aware of its power source's phase, controlmessages may be transmitted in the network based on the phase,accordingly. For instance, nodes may now provide phase-specificreporting to a data collection device (e.g., the root nodes). Also, amanagement device (e.g., within distribution center 410 or otherwise)may issue “demand response” or “DR” messages to the nodes/devices of thenetwork based on their phases. For example, as noted above in FIG. 5B,imbalance may occur in the polyphase system, such as from overloadedphases, etc., which can cause various concerns/issues.

As such, as shown in FIG. 10, a DR message 1000 may be issued on aparticular DAG (e.g., 900-2), and any node on that DAG (nodes B, E, andG) may invoke its DR process 248 to take the required action of the DRmessage. For instance, if too much power is being drawn from aparticular phase (e.g., from a substation), the DR message 1000 mayrequest that any non-necessary systems conserve energy (shut down,reduce consumption, delay operation, etc.) until the problem has beenresolved (e.g., until a second DR message 1000 clearing the condition,or else after a set time period indicated in the first DR message).Accordingly, devices may receive a phase-specific DR message on the DAGspecific to their particular phase, and may react/respond to the DRmessage as requested (if able to do so). Alternatively, such as where nophase-specific DAGs are created, the DR message 1000 may bephase-specific but sent to all devices on a general DAG, such that alldevices receive the message and determine whether to react based on theindicated specific phase.

Note that the techniques herein allow for the dynamic monitoring of thezero crossing timing with regard to the superframe, which thereforeallows for detection of phase changes at the devices. For instance, adevice may detect that it was moved to another phase based on the ACwaveform, and may then automatically invoke a DAG migration (RPL treemigration) to the DAG specific to the new phase. Note that a node mayhave policies that dictate when to change DAGs since DAG migration canbe expensive in terms of control plane messages, authentication,requirements, etc.

FIG. 11 illustrates an example physical change (e.g., a downed powerline) in the grid that could result in a device being moved to anotherphase as part of the restoration process. Accordingly, based on thetechniques described above, node H may correspondingly change DAGs to900-2 as shown in FIG. 12 to join the DAG specific to its new phase, L2.

FIG. 13 illustrates an example simplified procedure for creatingphase-based DAGs 900 in accordance with one or more embodimentsdescribed herein, from the perspective of each device 200 (e.g., joininga DAG). The procedure 1300 starts at step 1305, and continues to step1310, where a device, e.g., node H, monitor an AC waveform 600 of anelectrical power source, which is part of a polyphase power sourcesystem. In step 1315, the system determines a particular phase (e.g.,L1) of the polyphase power source system based on the AC waveform, e.g.,according to its relation to frequency hopping superframes and withrespect to the reference provided by the DAG root as described by theillustrative embodiment herein and also with reference to FIG. 14 below.

Once the device's particular phase is determined, then in step 1320 thedevice may join a DAG specific to the particular phase, e.g., DAG 900-1for phase L1. Note also that as described above, the device may alsojoin a corresponding multicast group on the DAG. If there is a phasechange detected in step 1325 (such as by monitoring and determining instep 1310 and 1315), then in step 1330 the device may invoke a DAGmigration to thus join a second DAG specific to the newly determinedphase. Also, if a DR message 1000 is received on the DAG in step 1335,then in step 1340 the device may react to the DR message as describedabove. In case no DR command is received in step 1335, the procedurereturns to step 1325 where it checks whether a topology change of thegrid occurred which may have resulted in changing the phase from whichthe specific node receives its power.

It should be noted that while certain steps within procedure 1300 may beoptional as described above, the steps in FIG. 13 are merely an examplefor illustration, and certain steps may be included or excluded asdesired.

FIG. 14 illustrates an example simplified procedure for determining aparticular phase of a device in a polyphase system based on its relationto frequency hopping in accordance with one or more embodimentsdescribed herein. The procedure 1400 starts at step 1405, and continuesto step 1410, where the device, e.g., node H, monitors an AC waveform600-1 of an electrical power source of a polyphase power source system.In step 1415, the device/system detects a time of a zero crossing 610-1of the AC waveform, and correlates it with frequency hopping in step1420 to determine a particular phase of the polyphase power sourcesystem at the device based on the time of the zero crossing relative toa corresponding location within a frequency hopping superframe 710, asdescribed in greater detail above. Note that at this point, the devicemay or may not join a corresponding DAG as mentioned above.

In the event a DR message 1000 is received at the device in step 1425,then assuming no DAG is joined, in step 1530 the device may determinewhether the DR message was intended for its particular phase (or a groupof phases or all phases). If so, then in step 1435 the device may reactto the DR message, accordingly. The procedure 1400 returns to step 1410to continue to monitor the waveform in order to detect any changes tothe device's phase.

It should also be noted that while certain steps within procedure 1400may be optional as described above, the steps in FIG. 14 are merely anexample for illustration, and certain steps may be included or excludedas desired. Further, while procedures 1300 and 1400 are describedseparately, certain steps from each procedure may be incorporated intoeach other procedure, and the two procedures are not meant to bemutually exclusive.

The novel techniques described herein, therefore, provide forphase-based operation/control of devices in a computer network that arepowered by a polyphase electric system. In particular, the noveltechniques create DAGs dynamically based on which phase is powering aparticular device, which in one or more illustrative embodiments isbased on the timing of the phase with relation to frequency hoppingsuperframes. Accordingly, control of the devices in the computer networkmay be phase-based, such that messages may be delivered to (and receivedfrom) specific devices in the network based on their respective phases,such as DR messages or other types of management messages. That is, bysegmenting the DAG according to the specific phase from which thedevices draw energy, phase-targeted messages (such as a DR) can traversethe LLNs without affecting traffic to/from devices connected to otherphases. Further, the adaptive (dynamic) techniques above providefunctionality that would be difficult, if not practically impossible, toperform manually, particularly for networks with a large number ofnodes.

While there have been shown and described illustrative embodiments thatprovide for phase-based operation/control of devices in a computernetwork that are powered by a polyphase electric system, it is to beunderstood that various other adaptations and modifications may be madewithin the spirit and scope of the embodiments herein. For example, theembodiments have been shown and described herein with relation to LLNs,and more particular, to the RPL protocol. However, the embodiments intheir broader sense are not as limited, and may, in fact, be used withother types of networks and/or protocols utilizing DAG routing (e.g.,distance vector protocols). Also, while the embodiments above generallydescribe the polyphase source system as a three-phase system, this ismerely one example embodiment of a polyphase system (granted, the mostprevalent type today), and is not meant to limit the embodiments herein.In addition, as noted above, while the techniques above may be madereference to transmission systems or distribution systems, however thedisclosure herein applies to both the transmission and distributionportions of the electric grid.

Further, while certain embodiments above described joining a particularDAG based on phase, or changing to a new DAG based on phase changes,there may be instances where a phase-based DAG decision is unavailable.For instance, if a device is powered by a particular phase, but thereare no parent nodes available in a DAG of that phase, then the devicemay be forced to join a DAG of a different phase. In this case, thedevice may configure itself to ignore certain control messages (e.g., DRmessages), and/or may notify its DAG root of the mismatched phase forfurther processing and/or reporting. Typically, however, sincephase-based electric power distribution generally creates regions oflike-phased devices (e.g., a building, a neighborhood, etc.), thelikelihood of a device being separated from a DAG of its phase islimited. One instance where phase-based decisions may not be available(or necessary), is where battery-operated devices share the computernetwork 100 with grid-powered (plug-in) devices, in which case thebatter-operated devices may join whichever DAG is more convenient.

Moreover, while the techniques above describe a distributed phase-baseddetermination, where each device determines its own phase, it ispossible in one or more embodiments herein to provide a centralizedcomputation, where devices learn of their phases from an externalsource. For instance, each device in the network may send information toa central computation device (e.g., a DAG root on a genericall-encompassing DAG), such as information regarding its detected zerocrossings, and then the root device may respond to the device with thedevice's associated phase. Also, those skilled in the art shouldrecognize that the zero crossing technique used for identifying thephase to which a device is connected is merely one illustrativeembodiment. The embodiments herein may also alternatively (or inaddition) use systems which incorporate other phase detection methods,such as external devices (e.g., “phasors” which operate in sync with theGPS clocks).

The foregoing description has been directed to specific embodiments. Itwill be apparent, however, that other variations and modifications maybe made to the described embodiments, with the attainment of some or allof their advantages. For instance, it is expressly contemplated that thecomponents and/or elements described herein can be implemented assoftware being stored on a tangible (non-transitory) computer-readablemedium (e.g., disks/CDs/etc.) having program instructions executing on acomputer, hardware, firmware, or a combination thereof. Accordingly thisdescription is to be taken only by way of example and not to otherwiselimit the scope of the embodiments herein. Therefore, it is the objectof the appended claims to cover all such variations and modifications ascome within the true spirit and scope of the embodiments herein.

What is claimed is:
 1. A method, comprising: monitoring analternating-current (AC) waveform of an electrical power source at adevice in a computer network, the power source being part of a polyphasepower source system; determining a particular phase of the polyphasepower source system at the device based on the AC waveform; and inresponse to determining the particular phase of the device, joining, bythe device, a directed acyclic graph (DAG) specific to the particularphase until the device detects that the particular phase of the deviceshas changed.
 2. The method as in claim 1, wherein determining theparticular phase comprises: detecting a time of a zero crossing of theAC waveform; and determining the particular phase based on the time ofthe zero crossing relative to a corresponding location within afrequency hopping superframe of the computer network.
 3. The method asin claim 1, further comprising: receiving a demand response (DR) messageon the DAG specific to the particular phase; and reacting to the DRmessage.
 4. The method as in claim 1, further comprising: detecting aphase change at the device based on the AC waveform to a second phase;and invoking a DAG migration by the device to a second DAG specific tothe second phase.
 5. The method as in claim 1, further comprising:joining a multicast group established on the DAG, the multicast groupspecific to the particular phase.
 6. The method as in claim 1, wherein avirtual local area network (VLAN) is configured on the DAG.
 7. Themethod as in claim 1, further comprising: notifying an external sourceof the AC waveform at the device; and receiving a notification from theexternal source, wherein determining the particular phase of thepolyphase power source system at the device based on the notificationfrom the external source.
 8. An apparatus, comprising: a power supplyadapted to receive an alternating-current (AC) waveform of an electricalpower source, the power source being part of a polyphase power sourcesystem; a network interface adapted to communicate in a computernetwork; a processor coupled to the network interface and adapted toexecute one or more processes; and a memory configured to store aprocess executable by the processor, the process when executed operableto: monitor the AC waveform; determine a particular phase of thepolyphase power source system at the power supply based on the ACwaveform; and in response to a determination of the particular phase ofthe device, join a directed acyclic graph (DAG) specific to theparticular phase via the network interface until the device detects thatthe particular phase of the devices has changed.
 9. The apparatus as inclaim 8, wherein the process when executed to determine the particularphase is operable to: detect a time of a zero crossing of the ACwaveform; and determine the particular phase based on the time of thezero crossing relative to a corresponding location within a frequencyhopping superframe of the computer network used by the networkinterface.
 10. The apparatus as in claim 8, wherein the process whenexecuted to is further operable to: receive a demand response (DR)message on the DAG specific to the particular phase; and react to the DRmessage.
 11. The apparatus as in claim 8, wherein the process whenexecuted to is further operable to: detect a phase change at the powersupply based on the AC waveform to a second phase; and invoking a DAGmigration by the apparatus to a second DAG specific to the second phase.12. A tangible, non-transitory, computer-readable media having softwareencoded thereon, the software when executed by a processor on a devicein a computer network operable to: monitor an alternating-current (AC)waveform of an electrical power source at the device, the power sourcebeing part of a polyphase power source system; determine a particularphase of the polyphase power source system at the device based on the ACwaveform; and in response to a determination of the particular phase ofthe device, join the device to a directed acyclic graph (DAG) specificto the particular phase until the device detects that the particularphase of the devices has changed.
 13. The computer-readable media as inclaim 12, wherein the software when executed to determine the particularphase is operable to: detect a time of a zero crossing of the ACwaveform; and determine the particular phase based on the time of thezero crossing relative to a corresponding location within a frequencyhopping superframe of the computer network.
 14. The computer-readablemedia as in claim 12, wherein the software when executed is furtheroperable to: receive a demand response (DR) message on the DAG specificto the particular phase; and react to the DR message.
 15. Thecomputer-readable media as in claim 12, wherein the software whenexecuted is further operable to: detect a phase change at the devicebased on the AC waveform to a second phase; and invoke a DAG migrationby the device to a second DAG specific to the second phase.